A particle that barely ranks as a footnote in a physics text may be about to lift the cleanup of the stricken Fukushima Daiichi nuclear complex in Japan over a crucial obstacle.

Inside the complex, there are three wrecked reactor cores, twisted masses of hundreds of tons of highly radioactive uranium, plutonium, cesium and strontium. After the meltdown, which followed a tsunami and earthquake in 2011, most of the material in the plant’s reactors resolidified, in difficult shapes and in confined spaces, wrapped around and through the structural parts of the reactors and the buildings.

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Or at least, that is what the engineers think. Nobody really knows, because nobody has yet examined many of the most important parts of the wreckage. Though three and a half years have passed, it is still too dangerous to climb inside for a look, and sending in a camera would risk more leaks. Engineers do not have enough data to even run a computer model that could tell them how much of the reactor cores are intact and how much of them melted, because the measurement systems inside the buildings were out of commission for days after the accident.

And though the buildings may be leaking, they were built of concrete and steel so thick that there is no hope of using X-rays or other conventional imaging technology to scan the wreckage from a safe distance.

To clean up the reactors, special tools must be custom-made, according to Duncan W. McBranch, the chief technology officer at Los Alamos National Laboratory, and the tools “can be much better designed if you had a good idea of what’s inside.” But “nobody knows what happened inside,” he said. “Nobody wants to go in to find out.”

That is where muons come in.

In the next few days, Toshiba, the contractor in charge of the initial cleanup work, and the laboratory expect to sign a formal agreement to deploy a new technology that experts believe will yield three-dimensional images of the wrecked reactor cores, and will be able to differentiate the uranium and plutonium from other materials, even when 10 feet of concrete and steel are in the way.

The Energy Department has been working on the technology for years, and already licenses it in a less advanced form for a more limited job: A Virginia company is using it in a device that screens shipping containers for smuggled uranium or plutonium that could be used in a nuclear bomb. The lab’s new version will be much more ambitious and will focus on mapping rather than just detection.

The technique takes advantage of the fact that everything on earth is constantly being bombarded by muons, subatomic particles that are somewhat like electrons, though about 200 times as heavy. Muons are shaken loose from molecules in the atmosphere by cosmic radiation, and rain down on the earth. They are so penetrating that most go straight through the planet and zip out the other side at near the speed of light without any effect on their trajectory.

Japan is increasingly turning to other countries for the technology needed to clean up Fukushima. This month, Tepco, the utility that operated the power plant, announced a deal with Kurion, a waste-handling company based in Irvine, Calif., for a mobile system to scrub radioactive strontium from 340,000 tons of contaminated water at the site.

Lake H. Barrett, an engineer who is not directly involved in the muon project, said the technique was certainly worth trying. Mr. Barrett was the director of the Nuclear Regulatory Commission office on site at the cleanup of the Three Mile Island nuclear plant near Harrisburg, Pa.; he is now an adviser to the president of Tepco.

Referring to the technology’s use in detecting smuggled weapons fuel, he said, “It’s nice to see the synergy of nonproliferation technologies, on which we in the U.S. have spent hundreds of millions of dollars, applied to another area.”

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